diff --git a/src/compiler/scala/tools/reflect/FastStringInterpolator.scala b/src/compiler/scala/tools/reflect/FastStringInterpolator.scala
index 7f664aa85b29..0d655b266681 100644
--- a/src/compiler/scala/tools/reflect/FastStringInterpolator.scala
+++ b/src/compiler/scala/tools/reflect/FastStringInterpolator.scala
@@ -106,11 +106,22 @@ trait FastStringInterpolator extends FormatInterpolator {
       val emptyLit = treatedContents.isEmpty
       if (i < numLits - 1) {
         val arg = argsIndexed(i)
-        if (linting && !(arg.tpe =:= definitions.StringTpe))
+        def warn(msg: String) = runReporting.warning(arg.pos, msg, WFlagTostringInterpolated, c.internal.enclosingOwner)
+        def stringlyBranches = arg match {
+          case If(_, thenp, elsep) => thenp.tpe <:< definitions.StringTpe && elsep.tpe <:< definitions.StringTpe
+          case Match(_, cases) =>
+            cases.forall {
+              case CaseDef(_, _, body) => body.tpe <:< definitions.StringTpe
+            }
+          case _ => false
+        }
+        if (linting && !(arg.tpe =:= definitions.StringTpe)) // literals are widened, so include Null, Nothing
           if (arg.tpe.typeSymbol eq definitions.UnitClass)
-            runReporting.warning(arg.pos, "interpolated Unit value", WFlagTostringInterpolated, c.internal.enclosingOwner)
+            warn("interpolated Unit value")
+          else if ((arg.tpe.typeSymbol eq definitions.AnyClass) && stringlyBranches)
+            () // if or match assumes expected type Any of interpolated expression, check branches are strings
           else if (!definitions.isPrimitiveValueType(arg.tpe))
-            runReporting.warning(arg.pos, "interpolation uses toString", WFlagTostringInterpolated, c.internal.enclosingOwner)
+            warn("interpolation uses toString")
         concatArgs += arg
       }
       if (!emptyLit) concatArgs += lit
diff --git a/test/files/neg/tostring-interpolated.check b/test/files/neg/tostring-interpolated.check
index 13d0527e6745..2de2aecd4e78 100644
--- a/test/files/neg/tostring-interpolated.check
+++ b/test/files/neg/tostring-interpolated.check
@@ -1,41 +1,42 @@
-tostring-interpolated.scala:7: error: interpolation uses toString
-Applicable -Wconf / @nowarn filters for this fatal warning: msg=<part of the message>, cat=w-flag-tostring-interpolated, site=T.f
+tostring-interpolated.scala:8: warning: interpolation uses toString
   def f = f"$c" // warn
              ^
-tostring-interpolated.scala:8: error: interpolation uses toString
-Applicable -Wconf / @nowarn filters for this fatal warning: msg=<part of the message>, cat=w-flag-tostring-interpolated, site=T.s
+tostring-interpolated.scala:9: warning: interpolation uses toString
   def s = s"$c" // warn
              ^
-tostring-interpolated.scala:9: error: interpolation uses toString
-Applicable -Wconf / @nowarn filters for this fatal warning: msg=<part of the message>, cat=w-flag-tostring-interpolated, site=T.r
+tostring-interpolated.scala:10: warning: interpolation uses toString
   def r = raw"$c" // warn
                ^
-tostring-interpolated.scala:11: error: interpolation uses toString
-Applicable -Wconf / @nowarn filters for this fatal warning: msg=<part of the message>, cat=w-flag-tostring-interpolated, site=T.format
+tostring-interpolated.scala:12: warning: interpolation uses toString
   def format = f"${c.x}%d in $c or $c%s" // warn using c.toString // warn
                               ^
-tostring-interpolated.scala:11: error: interpolation uses toString
-Applicable -Wconf / @nowarn filters for this fatal warning: msg=<part of the message>, cat=w-flag-tostring-interpolated, site=T.format
+tostring-interpolated.scala:12: warning: interpolation uses toString
   def format = f"${c.x}%d in $c or $c%s" // warn using c.toString // warn
                                       ^
-tostring-interpolated.scala:13: warning: Boolean format is null test for non-Boolean
-  def bool = f"$c%b" // warn just a null check
-                  ^
-tostring-interpolated.scala:15: error: interpolation uses toString
-Applicable -Wconf / @nowarn filters for this fatal warning: msg=<part of the message>, cat=w-flag-tostring-interpolated, site=T.oops
-  def oops = s"${null} slipped thru my fingers" // warn
-                 ^
-tostring-interpolated.scala:20: error: interpolation uses toString
-Applicable -Wconf / @nowarn filters for this fatal warning: msg=<part of the message>, cat=w-flag-tostring-interpolated, site=T.greeting
+tostring-interpolated.scala:17: warning: interpolation uses toString
   def greeting = s"$sb, world" // warn
                     ^
-tostring-interpolated.scala:31: error: interpolated Unit value
-Applicable -Wconf / @nowarn filters for this fatal warning: msg=<part of the message>, cat=w-flag-tostring-interpolated, site=Mitigations.unitized
+tostring-interpolated.scala:21: warning: Boolean format is null test for non-Boolean
+  def bool = f"$c%b" // warn just a null check (quirk of Java format)
+                  ^
+tostring-interpolated.scala:23: warning: interpolation uses toString
+  def oops = s"${null} slipped thru my fingers" // warn although conforms to String
+                 ^
+tostring-interpolated.scala:25: warning: interpolation uses toString
+  def exceptionally = s"Hello, ${???}" // warn although conforms to String
+                                 ^
+tostring-interpolated.scala:36: warning: interpolated Unit value
   def unitized = s"unfortunately $shown" // warn accidental unit value
                                   ^
-tostring-interpolated.scala:32: error: interpolated Unit value
-Applicable -Wconf / @nowarn filters for this fatal warning: msg=<part of the message>, cat=w-flag-tostring-interpolated, site=Mitigations.funitized
+tostring-interpolated.scala:37: warning: interpolated Unit value
   def funitized = f"unfortunately $shown" // warn accidental unit value
                                        ^
-1 warning
-9 errors
+tostring-interpolated.scala:53: warning: interpolation uses toString
+    val greeting = s"Hello ${if (shouldCaps) "WORLD" else world}" // warn
+                             ^
+tostring-interpolated.scala:64: warning: interpolation uses toString
+    val greeting = s"Hello ${x match { case 42 => "WORLD" case 27 => world case _ => ??? }}" // warn
+                               ^
+error: No warnings can be incurred under -Werror.
+13 warnings
+1 error
diff --git a/test/files/neg/tostring-interpolated.scala b/test/files/neg/tostring-interpolated.scala
index 959720e4aacb..5f44897ece39 100644
--- a/test/files/neg/tostring-interpolated.scala
+++ b/test/files/neg/tostring-interpolated.scala
@@ -1,4 +1,5 @@
-//> using options -Wconf:cat=w-flag-tostring-interpolated:e -Wtostring-interpolated
+//> using options -Werror -Wtostring-interpolated
+///> using options -Wconf:cat=w-flag-tostring-interpolated:e -Wtostring-interpolated
 
 case class C(x: Int)
 
@@ -10,21 +11,25 @@ trait T {
 
   def format = f"${c.x}%d in $c or $c%s" // warn using c.toString // warn
 
-  def bool = f"$c%b" // warn just a null check
-
-  def oops = s"${null} slipped thru my fingers" // warn
-
   def ok = s"${c.toString}"
 
   def sb = new StringBuilder().append("hello")
   def greeting = s"$sb, world" // warn
+
+  def literally = s"Hello, ${"world"}" // nowarn literal, widened to String
+
+  def bool = f"$c%b" // warn just a null check (quirk of Java format)
+
+  def oops = s"${null} slipped thru my fingers" // warn although conforms to String
+
+  def exceptionally = s"Hello, ${???}" // warn although conforms to String
 }
 
 class Mitigations {
 
   val s = "hello, world"
   val i = 42
-  def shown() = println("shown")
+  def shown = println("shown") // nowarn parenless Unit def because that is at refchecks
 
   def ok = s"$s is ok"
   def jersey = s"number $i"
@@ -34,3 +39,34 @@ class Mitigations {
   def nopct = f"$s is ok"
   def nofmt = f"number $i"
 }
+
+class Branches {
+
+  class C {
+    val shouldCaps = true
+    val greeting = s"Hello ${if (shouldCaps) "WORLD" else "world"}"
+  }
+
+  class D {
+    val shouldCaps = true
+    object world { override def toString = "world" }
+    val greeting = s"Hello ${if (shouldCaps) "WORLD" else world}" // warn
+  }
+
+  class E {
+    def x = 42
+    val greeting = s"Hello ${x match { case 42 => "WORLD" case 27 => "world" case _ => ??? }}"
+  }
+
+  class F {
+    def x = 42
+    object world { override def toString = "world" }
+    val greeting = s"Hello ${x match { case 42 => "WORLD" case 27 => world case _ => ??? }}" // warn
+  }
+
+  class Z {
+    val shouldCaps = true
+    val greeting = s"Hello ${if (shouldCaps) ??? else null}" // nowarn quirk, no string result
+  }
+
+}